1. Field of the Invention
The present invention relates to a printer and a thermal printing method, more particularly to a printer and a thermal printing method, for recording a color image on a thermal recording material at a single conveyance by use of plural thermal heads.
2. Description of the Prior Arts
It is used a color thermo-sensitive recording paper in a color thermal printer that has at least three thermo-sensitive coloring layers to be developed color by applying heat. The color thermo-sensitive recording paper is heated to record a full-color image thereon by the thermal head, as the color thermo-sensitive recording paper and the thermal head are relatively moved. The color thermo-sensitive paper is constituted by successively forming at least a cyan thermal coloring layer, a magenta thermal coloring layer, and a yellow thermal coloring layer on a substrate. Each thermal coloring layer has different thermal sensitivity so as to make each thermal coloring layer develop color selectively. The cyan thermal coloring layer of bottom layer is the lowest in the thermal sensitivity, whereas the yellow thermal coloring layer of top layer is the highest. The thermal recording is conducted in order from the yellow thermal coloring layer of the highest thermal sensitivity. An electromagnetic ray proper to the recorded thermal coloring layer is irradiated for optical fixation before the recording of the following thermal coloring layer, preventing the recorded thermal coloring layer from developing color again.
There are a one-head three-pass type and a three-head one-pass type in the color thermal printer. In the one-head three-pass type it is used a single thermal head that moves the color thermo-sensitive recording paper back and forth three times, to record a color image by three color frame sequential recording. In the three-head one-pass type a yellow thermal head, a magenta thermal head, and a cyan thermal head are disposed at suitable intervals from upstream along the conveyance path of the color thermo-sensitive recording paper. A yellow ultraviolet rays lamp is disposed between the yellow thermal head and the magenta thermal head. And a magenta ultraviolet rays lamp is disposed between the magenta thermal head and the cyan thermal head.
A yellow color image is heat-recorded on the yellow thermal coloring layer of the top layer by the yellow thermal head while conveying the color thermo-sensitive recording paper from upstream to downstream. After the heat recording of the yellow color image, the ultraviolet rays are irradiated from the yellow ultraviolet rays lamp, fixing the yellow thermal coloring layer optically. Then a magenta color image is heat-recorded on the magenta thermal coloring layer by the magenta thermal head. The magenta thermal head produces heat energy larger than the yellow thermal head. After the heat recording of the magenta color image, the ultraviolet rays are irradiated from the magenta ultraviolet rays lamp, fixing the magenta thermal coloring layer optically. And finally, a cyan color image is heat-recorded on the cyan thermal coloring layer by the cyan thermal head. The yellow color image, the magenta color image, and the cyan color image are sequentially heat-recorded and optical-fixed while conveying the color thermo-sensitive recording paper from upstream to downstream, forming a full color image on the color thermo-sensitive recording paper.
A thermal head of the same structure is used in the thermal printer of the three-head one-pass type for the heat recording on each thermal coloring layer. Since the respective thermal coloring layers are different in depth, a difference is created in the isothermal curve of each thermal coloring layer, varying dot size that due to color. Therefore, the dot size of the yellow thermal coloring layer of the top layer becomes large, whereas that of the cyan thermal coloring layer of the bottom layer becomes small. The dot size is different in each three color, resulting in color blurring of the black letter or a reduction in printing resolution.
An object of the present invention is to provide a printer and a thermal printing method that equalizes the dot size of each color.
Another object of the present invention is to provide a printer and a thermal printing method that consist of simple structure for equalizing the dot size of each color.
And final object of the present invention is to provide a printer and a thermal printing method that prevent occurrence of the color blurring and a reduction in the printing resolution.
To achieve the above and other objects, according to the thermal printing method of the present invention, the lower the thermal coloring layer is, the wider the isothermal curve is widened in the surface of a thermal recording material. That is, a cyan thermal head for recording a cyan thermal coloring layer of the bottom layer is widened the area for heating the color thermo-sensitive recording paper in comparison with a yellow thermal head for recording a yellow thermal coloring layer of the top layer. A magenta thermal head for recording a magenta thermal coloring layer of the middle layer is the middle of the cyan thermal head and the yellow thermal head.
According to the thermal printer of the present invention, the heating element of the cyan thermal head is longer than that of the yellow thermal head in a sub scanning direction. According to another embodiment of the present invention, the partial glaze of the cyan thermal head is longer than that of the yellow thermal head in the radius of curvature. According to the third embodiment of the present invention, the partial glaze of the cyan thermal head is thicker than that of the yellow thermal head. According to the fourth embodiment of the present invention, the cyan platen rollers which nip the color thermo-sensitive recording paper with the cyan thermal head has a larger diameter in comparison with the yellow platen rollers. And finally according to the fifth embodiment, the cyan thermal head is cooled stronger than the yellow thermal head.
According to the present invention, the longer the distance between the thermal head and the thermal-sensitive coloring layer, the wider the isothermal curve becomes. Therefore it is possible to equalize the size of dot for each color.